DE3521477C2 - Lens for optical recording disks - Google Patents

Abstract

The objective for optical recording disks consists of a single lens, at least one surface of which is aspherical, according to the following formula: $F1 It is characterized by the fulfillment of the following conditions: (1) 0.1 < n-2 r1/f < 0.5 (2) 0.25 < d/f < 0.75 and, if the second lens surface is aspherical, advantageously also by (3) 0.7 < r1/f < 0.85 (4) 1.8 < n.

Description

Description

The invention relates to a lens for optical recording disks, consisting of a single lens, one surface of which is aspherical, according to the following formula

+ By ² + Ey ⁴ +Fy*+Gy> +Hy ¹⁰ + Iy"+ Jy"

r the vertex radius of this surface

&khgr; the distance of a point on the lens surface

to the tangential plane at the vertex and y the distance from this point to the optical axis

B, EF C, H, I, J denote the 2nd, 4th, 6th, 8th, 10th, 12th, 14th order coefficients of the aspherical surface and k the cone constant.

From DE-OS 33 26 196 and JP-OS 57-76 512, lenses for disks with optical recording are known which contain a lens with a convex surface on the light entry side, the two lens surfaces of which are designed as 2&uacgr; aepuarc surfaces.

US-PS 44 15 238 and JP-OS 58-68 711 disclose objectives for optical recording disks consisting of a lens, one surface of which is aspherical and characterized by coefficients up to the sixth order, and the other surface of which is spherical.

However, when the processability and other conditions for manufacturing aspherical lenses are taken into account, it is disadvantageous if the choice of lens materials is limited, as is the case with the known lenses.

The present invention is based on the object of specifying a lens for optical recording disks from a lens whose one surface is designed as an aspherical surface, which is light in weight and low in cost while satisfactorily correcting the aberrations. This is achieved by the features characterized in the claims.

The invention will now be explained in more detail with reference to the drawings. In the drawings Fig. 1 is a sectional view of a lens according to the invention Rg. 2 to 7 Correction curves of the objectives 1 to 6 according to the present invention.

The general requirements for an objective for optical recording disks are as follows: the focal length should be 4 to 5 mm for an infinitely distant object, the numerical aperture between 0.4 and 0.5, the refractive index and the thickness of the cover glass should be 1.58 at 800 nm wavelength and 1.2 mm respectively, the working distance should be a minimum of 2 mm and the aberrations should be corrected so that the mean square root of the wave aberration from the axis to the off-axis at half the angle of view of 1.245° is less than λ/14.

Especially when the lens in an optical recording disk lens is very small, the machinability of the material is more important than the material weight or unit cost. Therefore, lens manufacturing from low-refractive-index plastic or molding from glass is suitable for forming aspherical surfaces. However, there are many problems limiting the shape of an aspherical surface or a spherical surface, such as thickness, material, etc., which arise from technical difficulties associated with the machinability of the material.

The lenses for optical recording disks according to the invention can use a wide variety of materials, i.e. materials with a low and high refractive index, and satisfy the above-mentioned requirements even if only one lens is present in which only one surface is designed to be aspherical. It should also be mentioned that the asphere can be given to a surface with a large radius of curvature, since a material with a high refractive index can also be used.

As shown in Fig. 1, the objective lens for optical recording disks according to the invention consists of a single lens, one surface of which is formed aspherically, the aspherical shape being expressed by the following formula: ₂

therein referred to eo

&khgr; the distance from a point on the lens surface to the tangential plane at the vertex

y is the distance of the point to the optical axis

k is the cone constant b5 r is the vertex curvature radius of the aspherical surface

B. £ FG H, I, /respectively the coefficients 2~<;r, 4th, 6th, 8th, 10th, 12th, 14th order. A single lens has already been proposed in which the

ÖD Zl <<//

corrected for extra-axial aberrations, but this is limited to a lens in which the radius of curvature of the aspherical surface is small. As a result of these restrictive conditions, a material with a low refractive index must be used to meet the requirements of an optical recording disk lens.

The lenses according to the invention meet the requirements placed on a lens for optical recording disks, although a conventional lens material (refractive index 1.45 to 2.0) can be used.

In order to correct the off-axis aberrations, it is necessary to correct the sine conditions. The correction of the sine condition is given by the lens shape determined by the radii of curvature η and η of the single lens, its thickness d and its refractive index η . Therefore, if the lower limit or the iu upper limit of the condition

0.1 < n- ^- < 0.2 (1)

is exceeded, the sine condition becomes unfavorable and it becomes impossible to correct the off-axis rays.

Even if the condition (1) is satisfied, it will be impossible to make the working distance large if the value d exceeds the upper limit of the condition

20 0.25 <d/f< 0.75 (2)

and it becomes possible to provide sufficient thickness at the edge portion of the lens if tf is less than the lower limit, both of which are unfavorable.

If the object-side surface of the lens (the first surface) is made aspherical, it becomes possible to reduce the axial aberration almost to zero. The shape of the curve with respect to the sine condition depends largely on the shape of the first surface. In order to correct the sine condition and correct off-axis aberrations, it is necessary that the radius of curvature η of the first surface satisfies condition (1). If λ exceeds the upper limit of condition (1) or falls below the lower limit, the sine condition becomes unfavorable and it becomes impossible to correct off-axis aberrations. If condition (1) is satisfied, the shape of the lens will be biconvex if the refractive index is less than 1.75 and meniscus-shaped if the refractive index is higher.

If the second surface, which is the image-side surface of the lens, is spherical, the aberrations due to this surface would be unfavorable. However, if the thickness of the lens is kept large, the rays pass through closer to the optical axis and the aberrations become more favorable. If the upper limit given by condition (2) is exceeded, it becomes impossible to keep the working distance large enough. On the other hand, if the lower limit is exceeded, it becomes impossible to make the thickness at the edge portion of the lens large enough and satisfactorily correct the off-axis aberrations. The off-axis aberrations can be corrected even better if the lower limit of condition (2) is set at 0.4, that is, within the following limits Iic<|t:

0.4 < d/f < 0.75

In this respect, lenses according to the invention for disks with optical recording are described in which* &eegr; the first surface is aspherical. However, if the first surface of the lens is spherical and the second surface is aspherical, it is also possible to specify a lens for disks with optical recording that solves the problem underlying the invention if conditions (1) and (2) are satisfied.

If the first surface is designed as a spherical surface, the aberrations from this surface become large, but they can be corrected if the second surface is designed aspherical.

If the upper limit of condition (1) is exceeded or the lower limit is not reached, the aberrations of the first surface become so large that they cannot be satisfactorily corrected even if the second surface is aspherical.

In the case where the aberrations of the first surface are corrected by the second, the higher from the optical axis the rays pass through the second surface, the better the correction of the aberrations becomes. Therefore, it is desirable that the thickness of the lens does not become so large. If the upper limit of condition (2) is exceeded, it becomes impossible to keep the working distance sufficiently large and to correct the aberrations satisfactorily because the thickness becomes too large. On the other hand, it is advantageous if the upper limit of d is set at 0.4, that is, that d is within the following limits:

60 0.25 <d/f< 0.4

which makes it easy to correct the aberrations using the second surface.

If the lower limit of condition (2) is exceeded, it becomes impossible to obtain the required thickness at the edge portion of the lens.

The shape of the first surface or its radius of curvature r\ have a significant influence on the spherical aberration and the sine condition. Therefore, aberration correction is easier if the radius of curvature η of the first surface satisfies the following condition:

0.7 < -^ < 035 (3)

The larger the radius of curvature η of the first surface, the smaller the aberrations of the

first surface £. However, if the upper limit given by condition (3) is exceeded |

the sine condition becomes unsatisfactory and the off-axis aberrations also become too large.

If the lower limit of condition (3) is exceeded, both spherical aberration and S

Sinus condition is also unfavorable.

In addition, it is advantageous to choose the refractive index of the lens so large that the following condition &iacgr;&ogr; is satisfied:

1.8 < π (4)

because the radii of curvature of the surfaces can then be kept larger.

It is therefore advantageous for the correction of aberrations to set the refractive index π as given by the condition,

large, since the vertex curvature radius of the second surface then becomes large. It should be noted that the

The vertex curvature radius can be kept larger if the second surface instead of the first surface

is aspherical

The lenses for optical recording disks according to the invention contain a single lens, whereby

whose aberrations are sufficiently corrected to meet the requirements of a lens for optical recording disks. These lenses are lighter in weight, cheaper and easier to manufacture compared to the known lenses which are made from a combination of only spherical lenses or from a lens whose two surfaces are aspherical. Especially since it is possible to choose lens materials from a wide range of refractive indices, a material with good Processability can be selected. If a material is chosen with a refractive index of at least 1.8, the design and production can be chosen even more freely, because, for example, it is then possible to make the standard radius of curvature of the aspherical surface large.

4 sheets of drawings

Claims (1)

Patent claims 1. Lens for optical recording discs, consisting of a single lens, one surface of which is aspherical, according to the following formula 1 + yi -(I + k){y/rY where 10 r the vertex radius of this surface &khgr; the distance of a point on the lens surface to the tangential plane at the vertex and y the distance from this point to the optical axis designated by B, E, F, G, H, I, /the coefficients of 2nd, 4th, 6th, 8th, 10th, 12th, 14th order of the aspherical surface and k of the cone constant, characterized by the following data ± 5%: Table 1 /■■ = 0.$282 (aspherical) d = 0.6522 π = 1.89502 _r2 = 6.9675 / = 1, WD = 0.4632, t = 0.2609 k = -0.543, B = C, E = 0.21273 X 10" ¹ F = 0.1284 X 10" ² G = -0.59402 X 10" ² H - -0.12514 X 10" ^: / = 0.94836 X 10~ ⁹ J= -0.72672 X 10"» π - ^- = 0.24 referred to therein &eegr; the vertex curvature radius of the first surface O is the radius of curvature of the second surface &eegr; the refractive index of the lens at 800 nm WD the working distance and / the thickness of the cover glass. 2. Lens for optical recording discs, consisting of a single lens, one surface of which is aspherical, according to the following formula x = ==^L·==+ By ² + Ey ⁴ + Fy ⁶ + Gy' + Hy ^>0 + Iy ¹² + Jy" l+Vl-d +k){y/rY where r is the vertex radius of this surface x is the distance of a point on the lens surface to the tangential plane at the vertex and y the distance from this point to the optical axis designated by B, E, F, G, H. I, /the coefficients of 2nd, 4th, 6th, 8th, 10th, 12th, 14th order of the aspherical surface and k of the cone constant, characterized by the following data ± 5%: S η = 0.7586 (aspherical) 35 21 477 1.76825 I
S d = 0.5217 η = Table 2 Often _r2 = 42.1426 t =0.2609 / = 1, WD = 0.5364, E = 0.20071 U A: = 1.2248, 5 = 0, G = -0.81498 x 10' ² F = 0.15674 X 10"' / = 0.14655 x 10» H= -0.12911 XKT ¹ &ldquor;--^ = 0.25 ß; J = 0.53678 X HT" &igr;*
'-■■?. referred to therein i &rgr;&Lgr; the vertex curvature radius of the first surface H /2 the radius of curvature of the surface % π is the refractive index of the lens at 800 nsn Il WZ? the working distance and 25 p t is the thickness of the cover glass. ti 3. Lens for optical recording discs, consisting of a single lens, one surface of which pf is aspherical, according to the following formula &psgr; 30 1 + Rv ² + F/ + Fv ⁶ + f?v ^g + Hv ⁱ⁰ + Av ¹² :gj l+Vl-(.l + k)(y/ry ^f where U35 % r the vertex radius of this surface M &khgr; the distance of a point on the longitudinal surface || to the tangential plane at the vertex and r ^; ; y the distance from this point to the optical axis ;"·■· 40 |- denotes with B ₁ EF, G, H, I, J the coefficients of 2nd, 4th, 6th, 8th, 10th, 12th, 14th order of the ■| aspherical surface and Jt of the cone constant, characterized by the following data ± 5%: I Table 3 . ^5 r, = 0.6796 (aspherical) d = 0.5435 π = 1.60838 r ₂ = -4.0499 so / = 1, WD" 0.5326, t = 0.2609 k = 1.678, Ä = 0, E = 0.40592 F = -0.23739 G = -0.8318 X 10" ² H = -0.12906 X 10" ¹ / = 0.14642 X 10" ^{8 J} = ^0.53203 referred to therein �L is the vertex curvature radius of the first surface 65 r ₂ is the radius of curvature of the second surface
η is the refractive index of the lens at 800 nm WD is the working distance and t is the thickness of the cover glass. 4. Lens for optical recording discs, consisting of a single lens, one surface of which is aspherical, according to the following formula 1 + vTh to where r the vertex radius of this surface &khgr; the distance of a point on the lens surface to the tangential plane at the vertex and y the distance from this point to the optical axis designated by B. £ F, G, H. I, /the coefficients of 2nd, 4th, 6th, 8th, 10th, 12th, 14th order of the aspherical surface and Ar of the cone constant, characterized by the following data ±5%: Table 4 r, = 0.6022 (aspherical) d = 0.5217 η = 1.48423 _r2 = 1.7734 / = 1, WD = 0.5525, / = 0.2609 /t= 0.2721, Ä = 0, £=-0.30981 F = -0.68736, G = -0.80047 H = -0.21653 X 10, / = 0.11728 x i0" ⁷ J =0.11148 XlO" ¹⁰ η --^- = 0.28 referred to therein &Pgr; the vertex curvature radius of the first surface &Ggr;2 the radius of curvature of the second surface &eegr; the refractive index of the lens at 800 nm WD the working distance and t the thickness of the cover glass. 5. Lens for optical recording discs, consisting of a single lens, one surface of which is aspherical, according to the following formula _{x =} - +JtJ+ES+rf+Gf+H^+l^ + Jv" 1 + Vl -(I + k)(y/rY where r the vertex radius of this surface &khgr; the distance of a point on the lens surface to the tangential plane at the vertex and y the distance from this point to the optical axis to denotes B. £ F, GHI /the coefficients of 2nd, 4th, 6th, 8th, 10th, 12th, 14th order of the aspherical surface and Arder cone constants, characterized by the following data ±5%: (I = 0.2826 η 35 21 477 r ₂ = 4.7846 (aspherical) Table 5 r, = 0.775 / = 1, WD = 0.66288 — k = 39.1393, 5 = 0, - 1.89502 F " -0.19998, H = -0.65269, t =0.2609 J = 0.21333 X ΛΩ' ³ E = 0.2887 referred to therein G - 0.35833 / =0.18261X10"' &eegr; - &mdash;&mdash; = 0.35 n the radius of curvature of the first surface r ₂ is the vertex curvature radius of the second surface &pgr; the refractive index of the lens at 800 nm WD the working distance and 25 t the thickness of the cover glass. 6. Lens for optical recording discs, consisting of a single lens, one surface of which is aspherical, according to the following formula x = &mdash;fUL·==+By ² + Ey*+ Fy*+Gv*+ Hy ¹⁰ +Iy ¹² + Jy" \+Vl-(l + k)(y/rY r the vertex radius of this surface 35 &khgr; the distance of a point on the lens surface to the tangential plane at the vertex and y the distance from this point to the optical axis B, E, F, C, H, IJ denote the coefficients of the 2nd, 4th, 6th, 8th, 10th, 12th, 14th order of the 40 aspherical surface and Jt the cone constant, characterized by the following data ±5%: Table 6 r, = 0.7578 45 d = 0.2826 η = 1.82199 r ₂ = 8.0779 (aspherical) / = 1, WD = 0.6668, t = 0.2609 k = -151.7316, B = Q, E = 0.25648 F = -0.278, G =0.31784 55 H = -0.65269 / = 0.18261 X 10"' _/ = ^0.21333 referred to therein n the radius of curvature of the first surface /~2 the vertex curvature radius of the second surface 65 &pgr; the refractive index of the lens at 80G nm WD the working distance and t the thickness of the cover glass.